The purpose of this proposal is to determine the associations among dietary, estrogen metabolism, and breast cancer. This will involve collaboration with researchers in the Nutrition and Cancer Unit of the International Agency for Research on Cancer in Lyon, France, on the European Prospective Investigation into Cancer and Nutrition (EPIC), a large prospective study of 480,000 people in nine countries. The specific hypotheses are: 1) women with increased plasma concentrations of specific estrogen metabolites (16a and 4 hydroxy metabolites) are at increased risk of breast cancer and other cancers of the reproductive tract; 2) consumption of specific foods is associated with reduced plasma concentrations of 16alpha and 4 hydroxy estrogens, and lowered risk of cancers of the reproductive tract; and 3) plasma concentrations of estrogen metabolites are associated with genetic polymorphisms in enzymes associated with estrogen metabolism. This study may identify new cancer risk biomarkers that could be used to identify high risk women and to develop dietary and lifestyle strategies for cancer prevention.